preformation n : a theory (popular in the 18th
century and now discredited) that an individual develops by simple
enlargement of a tiny fully formed organism (a homunculus) that
exists in the germ cell [syn: theory
of preformation]

Preformationism is the theory that all organisms were created at the
same time, and that succeeding generations grow from homunculi,
animalcules, or other
fully-formed but miniature versions of themselves that have existed
since the beginning of creation.

In the case of humans, philosophers and
theologians have conjectured that every individual existed as a
homunculus in Adam's
testicles (spermism) or
Eve's
ovaries (ovism).

Epigenesis, in
contrast, is based on the idea that "each embryo or organism is
gradually produced from an undifferentiated mass by a series of
steps and stages during which new parts are added." (Magner 2002,
p. 154)

Philosophical development

Pythagoras was
one of the first to elaborate a theory of generation, the biological
production of offspring. He advocated a patrocline theory whereas
males contributed the essential characteristics of their offspring
while females contributed only a material substrate. Pythagorus'
theory heavily inspired Aristotle, who
elaborated both upon preformation and epigenesis, two distinct
theories of generation. Subsequent theorists such as Galen, Realdo
Colombo and Girolamo
Fabrici built upon Aristotle's theories, which were prevalent
well into the 17th century.

Leeuwenhoek discovered that the origin of semen
was the testicles and was a committed preformationist and spermist.
He reasoned that the movement of spermatozoa was evidence of animal
life, which presumed a complex structure and, for human sperm, a
soul. (Friedman 79)

Ova were known in some
non-mammalian species, and semen was thought to spur the
development of the preformed organism contained therein. The theory
that located the homonculus in the egg was called ovism. But when spermatozoa were
discovered, a rival camp of spermists sprang up, claiming
that the homunculus must come from the male. In fact, the term
"spermatozoon," coined by Karl
Ernst van Baer, means "seed animals."

In the 18th century, some animalculists thought
that an animal's sperm behaved like the adult animal, and recorded
such observations. Some, but not all, preformationists at this time
claimed to see miniature organisms inside the sex cells. But about
this time, spermists began to use more abstract arguments to
support their theories.

Jean Astruc,
noting that parents of both sexes seemed to influence the
characteristics of their offspring, suggested that the animalcule
came from the sperm and was then shaped as it passed into the egg.
Buffon
and Pierre
Louis Moreau also advocated theories to explain this
phenomenon.